The polyurethane resins generally used for a magnetic recording medium include polyester polyurethane resins and polyether polyurethane resins, which are prepared by reacting polyester polyols or polyether polyols with diisocyanate compounds such as tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, or methaxylylene diisocyanate using a urethanated reaction, as disclosed, for example, in Japanese Patent Application (OPI) No. 137522/81 (the term "OPI" as used herein means "unexamined published patent application"). The polyester polyols are obtained by reacting organic dibasic acids such as adipic acid, dimerized linolenic acid, maleic acid, or the like with glycols such as ethylene glycol, propylene glycol, butylene glycol or diethylene glycol, or polyhydric alcohols such as trimethylol propane, hexantriol, glycerine, trimethylol ethane, or pentaerythritol. The polyether polyols are obtained by reacting organic dibasic acids such as adipic acid, dimerized linolenic acid, maleic acid, or the like with propylene oxide or ethylene oxide.
However, when a conventional polyurethane resin is used as a binder for a magnetic recording medium, alcohol components are aliphatic compounds such as the above described glycols or polyhydric alcohols, and the glass transition point (Tg) of the thus obtained polyurethane resins is low. Accordingly, durability of a magnetic recording medium is insufficient.
Recently, higher image quality of a magnetic recording medium has been required. For this purpose, it is necessary to realize very close contact between a surface of a magnetic layer and a video head or an audio head, therefore it is desired to improve surface property of a magnetic recording medium, and to more strikingly improve dispersibility of ferromagnetic fine particles. However, conventionally known urethane resins show poor dispersibility for ferromagnetic particles, and fully satisfactory electromagnetic properties cannot be obtained. When a surface property of a magnetic layer is improved, the friction coefficient of a running system of a video tape recorder increases, thereby increasing running tension. Therefore, a magnetic recording medium must have high running durability even under such harsh conditions. In this connection, a magnetic recording medium having not only good surface property of a magnetic layer and good dispersibility of ferromagnetic fine particles, but also high running durability cannot be prepared in accordance with a conventional method. On the other hand, as a magnetic recording medium is in many cases stored for a long time after recording, a binder and the like having less deterioration of a magnetic recording medium during storage has been desired. However, conventionally used polyester polyurethane of the aliphatic type has not been satisfactory in this respect.
As a result of thorough investigation to overcome the defects of conventional techniques, the inventors of the present invention have attained the present invention.